"Walter Roberson" <roberson@ibd.nrc-cnrc.gc.ca> wrote in message
news:g1hn6u$nhj$1@canopus.cc.umanitoba.ca...

> In this thread you did not ask about #if vs #ifdef . They do not mean
> the same thing. For example,
>
> #define FOO 0
> #ifdef FOO
> /* this section *will* be compiled, because FOO -is- defined. */
> #endif
> #if FOO
> /* this section will *not* be compiled, because FOO's value is 0,
> and #if 0 is false */
> #endif
> #ifdef if
> /* this section will *not* be compiled, because the macro if is -not-
> defined. */
> #endif
> #if !if
> /* this section *will* be compiled. In a #if line, after all known
> macros are expanded, all remaining identifiers have the value 0
> substituted, without any consideration as to whether the identifiers
> might be C keywords or library functions. With no macro named if,
> the line would be equivilent to #if !0 which is #if 1 which is true.
> */
> #endif

Priceless.

Bill